Measuring Noise Underwater

Units for Measuring Noise

Underwater sound is usually expressed using the logarithmic deciBel scale.
All of the levels of sound presented in this report are expressed in decibels
referenced to one micro-Pascal, that is, as dB re 1 µPa. If you are not
familiar with the use of the dB scale, you can read a short
review of the deciBel scale.

Parameters for Estimating Noise

In order to provide an objective and quantitative assessment of the degree
of any environmental effect and the range within which it will occur, or to
understand the quality and applicability of an underwater measurement of noise
for this purpose, it is necessary to understand three parameters. These are:

The Effect Threshold, the level of sound at which a particular effect, such as death, injury or avoidance of a species occurs

The first two parameters allow the sound level at all points in the water to
be specified, and in the current state of knowledge are best measured at sea,
although it is in principle possible to estimate the transmission loss using
numerical models. Usually, this data has to be extrapolated to situations other
than those in which the noise was measured; the usual method of modelling the
level is from the expression:

SPL = SL - N log(R) - aR

Where:

SPL = Sound Pressure Level

SL = Source Level

N = Transmission Loss

R = Distance from the source

a = Coefficient of absorption

An understanding of all three parameters allows an estimate of the range within
which there will be an environmental effect to be estimated. If the population
density of a particular species within the area is known, the number of
individuals of the species likely to be effected can be estimated.